Photodiodes are currently used in conjunction with real-time oscilloscopes; however the sampling rate is limited to 60 GS/s, which is capable of processing signals with up to 30 GHz bandwidth. Current optical sampling systems use short optical pulses for high-resolution sampling. These systems, however, cannot operate single-shot and therefore cannot sample short optical packets or work with non-repetitive waveforms.
Autocorrelation and cross-correlation may also be used, however the optical signal must be deduced from the result and a single shot measurement is not possible over record lengths longer than a few picoseconds.
Other known methods include Frequency Resolved Optical Gating (FROG) and Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER), However, the record length for single-shot characterization using these techniques is limited to a couple of picoseconds.
Characterizing ultrafast optical signals has far-reaching applications in many areas of science and technology such as ultrafast phenomena (M. van Kampen, C. Jozsa, J. T. Kohlhepp, P. LeClair, L. Lagae, W. J. M. deJonge, and B. Koopmans, “All-optical probe of coherent spin waves,” Phys. Rev. Lett. 88, 227201-1-4 (2002); R. W. Schoenlein, W. Z. Lin, and J. G. Fujimoto, “Femtosecond studies of nonequilibrium electronic processes in metals,” Phys. Rev. Lett. 58, 1680-1683 (1987)), terahertz spectroscopy (M. Tonouchi, “Cutting-edge terahertz technology,” Nature Photonics 1, 97-105 (2007)), and ultrahigh-bandwidth communications (C. Dorrer, “High-speed measurements for optical telecommunication systems,” IEEE J. Sel. Top. Quantum Electron. 12, 843-858 (2006); N. Yamada, H. Ohta, and S. Nogiwa, “Polarization-insensitive optical sampling system using two KTP crystals,” IEEE Photon. Technol. Lett. 16, 215-217 (2004); M. Westlund, P. A. Andrekson, H. Sunnerud, J. Hansryd, and J. Li, “High-performance optical-fiber-nonlinearity-based optical waveform monitoring,” J. Lightwave Technol. 20, 2012-2022 (2005); J. Li, M. Westlund, H. Sunnerud, B.-E. Olsson, M. Karlsson, and P. A. Andrekson, “0.5-Tb/s eye-diagram measurement by optical sampling using XPM-induced wavelength shifting in highly nonlinear fiber,” IEEE Photon. Technol. Lett. 16, 566-568 (2004); C. Dorrer, C. R. Doerr, I. Kang, R. Ryf, J. Leuthold, and P. J. Winzer, “Measurement of eye diagrams and constellation diagrams of optical sources using linear optics and waveguide technology,” J. Lightwave Technol. 23, 178-186 (2005)). The traditional optoelectronic approach for optical signal sampling uses high-speed detectors and sample-and-hold circuits, which cannot be applied to signals that have excessively large bandwidths. One approach proposed to overcome this bandwidth problem is based on performing a cross-correlation with a short optical pulse train (C. Dorrer, “High-speed measurements for optical telecommunication systems,” IEEE J. Sel. Top. Quantum Electron. 12, 843-858 (2006); N. Yamada, H. Ohta, and S. Nogiwa, “Polarization-insensitive optical sampling system using two KTP crystals,” IEEE Photon. Technol. Lett. 16, 215-217 (2004); M. Westlund, P. A. Andrekson, H. Sunnerud, J. Hansryd, and J. Li, “High-performance optical-fiber-nonlinearity-based optical waveform monitoring,” J. Lightwave Technol. 20, 2012-2022 (2005); J. Li, M. Westlund, H. Sunnerud, B.-E. Olsson, M. Karlsson, and P. A. Andrekson, “0.5-Tb/s eye-diagram measurement by optical sampling using XPM-induced wavelength shifting in highly nonlinear fiber,” IEEE Photon. Technol. Lett. 16, 566-568 (2004); C. Dorrer, C. R. Doerr, I. Kang, R. Ryf, J. Leuthold, and P. J. Winzer, “Measurement of eye diagrams and constellation diagrams of optical sources using linear optics and waveguide technology,” J. Lightwave Technol. 23, 178-186 (2005)). Various techniques for cross-correlation have been demonstrated and are based primarily on a form of nonlinear optical gating (N. Yamada, H. Ohta, and S. Nogiwa, “Polarization-insensitive optical sampling system using two KTP crystals,” IEEE Photon. Technol. Lett. 16, 215-217 (2004); M. Westlund, P. A. Andrekson, H. Sunnerud, J. Hansryd, and J. Li, “High-performance optical-fiber-nonlinearity-based optical waveform monitoring,” J. Lightwave Technol. 20, 2012-2022 (2005); J. Li, M. Westlund, H. Sunnerud, B.-E. Olsson, M. Karlsson, and P. A. Andrekson, “0.5-Tb/s eye-diagram measurement by optical sampling using XPM-induced wavelength shifting in highly nonlinear fiber,” IEEE Photon. Technol. Lett. 16, 566-568 (2004)). For example, nonlinear processes such as sum- and difference-frequency generation (N. Yamada, H. Ohta, and S. Nogiwa, “Polarization-insensitive optical sampling system using two KTP crystals,” IEEE Photon. Technol. Lett. 16, 215-217 (2004)), four-wave mixing (FWM) (M. Westlund, P. A. Andrekson, H. Sunnerud, J. Hansryd, and J. Li, “High-performance optical-fiber-nonlinearity-based optical waveform monitoring,” J. Lightwave Technol. 20, 2012-2022 (2005)), and cross-phase modulation (J. Li, M. Westlund, H. Sunnerud, B.-E. Olsson, M. Karlsson, and P. A. Andrekson, “0.5-Tb/s eye-diagram measurement by optical sampling using XPM-induced wavelength shifting in highly nonlinear fiber,” IEEE Photon. Technol. Lett. 16, 566-568 (2004)) have been used for sampling communication signals at data rates as high as 500 Gb/s. Cross-correlation based on coherent linear detection has also been studied (C. Dorrer, C. R. Doerr, I. Kang, R. Ryf, J. Leuthold, and P. J. Winzer, “Measurement of eye diagrams and constellation diagrams of optical sources using linear optics and waveguide technology,” J. Lightwave Technol. 23, 178-186 (2005)) and is architecturally similar to these nonlinear techniques. While such systems can achieve sub-picosecond sampling resolution, the sample points are far apart in time since they are determined by the sampling pulse period (longer than a nanosecond). As a result, the samples must be post-processed in order to reconstruct a repetitive waveform or the eye diagram corresponding to a digital data stream. Optical packets and non-repetitive optical waveforms cannot be characterized using these sampling techniques, and rapid fluctuations in the signal are difficult to monitor and characterize because each pump pulse arrival generates only a single point in the sampled waveform.
Several solutions have been proposed that are capable of characterizing arbitrary waveforms in a single shot. One solution is time-to-space conversion, which allows temporal sampling using an array of detectors (J.-H. Chung and A. M. Weiner, “Real-time detection of femtosecond optical pulse sequences via time-to-space conversion in the lightwave communications band,” J. Lightwave Technol. 21, 3323-3333 (2003); Y. Takagi, Y. Yamada, K. Ishikawa, S. Shimizu, and S. Sakabe, “Ultrafast single-shot optical oscilloscope based on time-to-space conversion due to temporal and spatial walk-off effects in nonlinear mixing crystal,” Jpn. J. Appl. Phys. 44, 6546-6549 (2005); P. C. Sun, Y. T. Mazurenko, and Y. Fainman, “Femtosecond pulse imaging: ultrafast optical oscilloscope,” J. Opt. Soc. Am. A 14, 1159-1170 (1997); M. A. Foster, R. Salem, D. F. Geraghty, A. C. Turner-Foster, M. Lipson, and A. L. Gaeta, “Silicon-chip-based ultrafast optical oscilloscope,” Nature 456, 81-84 (2008)). However, most of the demonstrated systems based on this approach have limited waveform record length, and it is difficult to achieve fast detector read-out rates, which makes them unsuitable for monitoring rapidly varying signals.
Another solution is based on creating several replicas of the pump signal (C. Dorrer, J. Bromage, and J. D. Zuegel, “High-dymanic-range single-shot cross-correlator based on an optical pulse replicator,” Opt. Express 16, 13534-13544 (2008)) or the input waveform (K.-L. Deng, R. J. Runser, I. Glesk, and P. R. Prucnal, “Single-shot optical sampling oscilloscope for ultrafast optical waveforms,” IEEE Photon. Technol. Lett., 10, 397-399 (1998)) in order to perform single-shot sampling. However, the sensitivity is limited by the number of the created replicas, which leads to a trade-off between sensitivity and the number of samples.
There is therefore a need in the art for ultrafast systems and methods for optical waveform sampling with sub-picosecond resolution, and with record lengths longer than 100 ps. There is also a need in the art for ultrafast optical waveform sampling that can be applied to non-repetitive signals, short optical packets, and single events. There is also a need in the art for an optical waveform sampling system that can be used to convert a low-speed sampling device into an ultrafast sampling system.
In another embodiment, the temporal stretching system provides single-shot sampling, which allows characterizing short optical packets and one-time optical events, and monitoring fast variations of the optical signal.
Citation or identification of any reference in Section 2, or in any other section of this application, shall not be considered an admission that such reference is available as prior art to the present invention.